1. Field of the Invention
This invention relates to emission control systems for automotive engines, and particularly to a metering valve adapted for use in such emission control systems.
2. Description of Prior Developments
Various emission control systems have been devised for controlling the escape of fuel vapors from vehicle fuel tanks. One such system shown in U.S. Pat. No. 3,683,597 includes a canister containing activated charcoal. The canister has lines leading to the vehicle fuel tank and to the engine induction system so that, during operation of the engine, fuel vapors collected in the canister are drawn into the engine for combustion in the engine cylinders.
U.S. Pat. No. 4,475,522 shows an engine emission control system that includes a solenoid valve between a vapor collection canister and the engine intake system. The flow of fuel vapor to the engine can be controlled by selective energization of the valve solenoid.
U.S. Pat. No. 4,703,737 shows an emission control system that includes a solenoid valve having two oppositely moving armature plungers, such that one of the plungers controls vapor flow from the fuel tank to a charcoal canister, while the other plunger controls vapor flow from the canister to the engine intake manifold.
U.S. Pat. No. 4,944,276 shows an engine emission control system wherein a solenoid valve is arranged in series flow relation with a pressure-responsive diaphragm valve so that the diaphragm valve partially regulates the vapor flow rate to the engine when the solenoid valve is in an open condition.
U.S. Pat. No. 5,069,188 shows another engine emission control system that includes a diaphragm flow regulator means in association with a solenoid valve. In this case, the diaphragm operator has a port formed therein for recirculating fuel vapor back to the solenoid valve under certain operating conditions.